IBM Plex Sans is a trademark of IBM and distributed under the SIL Open Font License, Version 1.1.
There is an option
hrbrthemes.loadfonts which -- if set to
TRUE -- will
extrafont::loadfonts() to register non-core fonts with R PDF & PostScript
devices. If you are running under Windows, the package calls the same function
to register non-core fonts with the Windows graphics device.
This will take care of ensuring PDF/PostScript usage. The location of the font directory is displayed after the base import is complete. It is highly recommended that you install them on your system the same way you would any other font you wish to use in other programs.